Toomey is the conservative Republican seeking Sen. Arlen Specter's seat. The ex-congressman's popularity among the GOP faithful prompted Specter to bolt from the party in April.

Sestak is the congressman who is defying influential Democratic Party leaders, including President Barack Obama, by challenging Specter in next May's primary.

Together, the two men likely to be Specter's toughest opponents in his bid for a sixth term are teaming up to stage a town hall-style meeting on health care this week in Allentown. Wednesday night's event at Muhlenberg College will draw a stark contrast between opposite ends of the political spectrum and help citizens understand the implications of the reform debate, both candidates said.

"This is pure good governance," Sestak insisted.

Sestak strongly advocates optional government-run insurance coverage as a key element of a comprehensive reform plan, like the pending House bill he helped write.

Their event together comes as Obama is struggling on his top domestic priority – a plan to provide health coverage to nearly 50 million Americans who lack it and to rein in soaring costs.

Congressional Republicans are all but united in opposition to the White House proposal, while the president is under fire from liberal Democrats for not advocating more strongly for a public insurance option. Public backing for the president's initiative has shrunk in opinion polls, and opponents have drowned out supporters at many public meetings on the subject.

Sestak and Toomey deny that their unorthodox collaboration is a political stunt. Sestak proposed the meeting following an e-mail skirmish on the issue with Toomey, who promptly accepted.

It was "one of those spur-of-the-moment things," Sestak said.

The two have little in common except a desire to oust the incumbent. Both candidates accuse the 79-year-old Specter of flip-flopping on a central element of the health care debate.

"They've made an informal pact" to go after Specter, said Terry Madonna, a professor and pollster at Franklin & Marshall College in Lancaster.

Toomey, 47, lost the 2004 GOP primary to Specter by about 17,000 votes out of 1 million cast. He faces competition for the 2010 Republican nomination from anti-abortion activist Peg Luksik.

Toomey's strong GOP poll numbers persuaded Specter to leave the party earlier this year, and a Quinnipiac University poll in July showed Toomey neck and neck with Specter in a hypothetical general election matchup.

Sestak, 57, is a retired Navy vice admiral from the Philadelphia suburbs – the highest-ranking former military officer ever elected to the House, where is serving his second term. In addition to Specter, he faces opposition in the Democratic primary from state Rep. Bill Kortz, a former U.S. Steel Corp. manager from western Pennsylvania.

Toomey said Specter was not invited to particpate because he might muddle the discussion.

"With Arlen Specter, you never know who's going to show up," said Toomey, who served as president of the Club for Growth, a Washington-based conservative group, for several years until he stepped down in April to run for Senate.

Sestak said Specter "has already reversed his position and followed my leadership."

In April, Specter ended a more than four-decade relationship with the Republican Party, saying it had grown too conservative for him to win the nomination in 2010. He initially angered some Democrats by expressing opposition to a government-run insurance option, but has since said he would support optional public coverage so long as the government-run plan is subject to the same rules as private insurers.

During Congress' August break, Specter has defended Obama's health care agenda at a series of town hall meetings around the state, sometimes amid angry outbursts from opponents.

Specter campaign spokesman Chris Nicholas declined to comment on the Sestak-Toomey event. He said the senator has appeared with Sestak several times and looks forward to similar appearances with Toomey after the GOP primary.

Wednesday's 90-minute discussion in Toomey's hometown will be moderated by Christopher Borick, a political-science professor and pollster at Muhlenberg. He said he will strive to keep Sestak and Toomey focused on health care.

"Some of the previous discussions that I've seen have been more of a shouting match and people doing a lot of talking and not listening," Borick said.